PGI compiler bug

I believe to have found a bug in pgfortran occurring in versions 19.10, 20.7, 20.9 and 21.1. I have found it during testing for !107 (merged) in the Langevin part.

Edit: In the chosen design of the Hamiltonian structure, the bug doesn't apply any more.

The attached file contains a minimal example.pgi_bug.F90